Chemical control of apple trees: how to manage

Modern apple culture is not characterized by the simple use of state-of-the-art phytosanitary products to control diseases, but rather by knowledge of the products' action on the plant's physiology and aspects related to fruit quality. Therefore, the distinction between fungicides should not be based solely on disease control efficiency, but should be made by evaluating the beneficial effects of certain molecules on productivity and production quality.

Aside from disease control, certain molecules have other positive characteristics, such as activities on enzymes, which can act on the plant's hormonal balance. More recently, the importance of the beneficial physiological effect on plant development of strobilurin-based fungicides has been highlighted, so their effect on fruit plants should be better studied. In recent years, research into the fungal properties of strobilurins has highlighted direct influences on plant physiological processes where these products were used, even in plants not infected or threatened by pathogens. This influence is referred to as the Physiological Effect. These physiological effects range from the so-called greening effect and amelioration of stressors in the field and under controlled conditions, to hormonal regulatory influences and carbon and nitrogen assimilation by the plant. Strobilurins are synthetic derivatives of natural products that have been shown to be suitable for integrated management in the control of plant diseases, and in addition to the fungicidal effect, these molecules promote physiological changes in several crops with a positive influence on productivity. These secondary effects include a darkening of the green of the leaves, delaying senescence, increasing the concentration of chlorophyll, proteins and biomass production, favoring the harvest rate (Venâncio et al, 1999). This provides better development to the plant, promoting physiological changes that allow better use of CO₂, an increase in the activity of the nitrate reductase enzyme, a higher chlorophyll content and a reduction in stress, associated with a reduction in ethylene production, thus allowing a longer lifespan of the leaves. Within the group of strobilurins, metiram + pyraclostrobin stands out as a fungicide with a broad spectrum of action and which also presents physiological effects on plants, associated with the product's mode of action, which acts on cellular respiration, on mitochondria, temporarily interfering in electron transport and, as a consequence, there is better use of CO₂, reducing energy expenditure, resulting in greater accumulation of carbohydrates, increased ethylene nitrase activity, thus allowing longer leaf area duration.

A phytosanitary treatment program involving four applications of metiram + pyraclostrobin was evaluated in relation to a program without the inclusion of pyraclostrobin. The quality of the fruits, the content of mineral nutrients in the fruits and leaves and other characteristics related to the physiology of the fruit were evaluated, such as incidence of bitter pit, russeting, quality of the fruits in post-storage and vitamin C content in the fruits.

Specific objectives:

The. Effect on nutrient content in fresh fruit pulp;

B. Effect on vitamin C levels.

w. Effect on leaf nutrient content;

d. Effect on production

It is. Effect on the incidence of russeting;

f. Effect on the color of the fruit epidermis;

g. Effect on fruit conservation;

H. Effect on effective fruiting;

i. Effect on disease incidence.

The work was carried out in the municipality of Fraiburgo, Santa Catarina (Latitude 26^o46'31" S and Longitude 51^o00'54" W), in the 2006/2007, 2007/2008, 2008/2009 and 2009/2010 cycles and in Vacaria, in Rio Grande do Sul, in the 2007/2008 cycle. The orchard, in Fraiburgo, was established in 2002 , with the cultivars Imperial Gala and Fuji Suprema, on the Marubakaido rootstock with M-9 intergraft.The orchard was divided into two blocks, with a minimum area of ​​1,1 hectare per block, with each block containing the two cultivars. In each block, a phytosanitary treatment program was adopted, with the standard program being used in apple cultivation and the program involving the use of four applications of metiram + pyraclostrobin. In Vacaria the same system was adopted, however, the Gala cultivar was used The other activities carried out in the orchard were the same in both blocks.

Results

In the Imperial Gala and Fuji Suprema cultivars, an increase in calcium levels in the fruit pulp at pre-harvest was observed, with an increase of 4% to 18,9% in relation to the standard treatment, depending on the year. In the Fuji Suprema cultivar, Ca levels in the fruit pulp at pre-harvest showed an increase in calcium levels lower than those observed in the Imperial Gala cultivar. This difference between the Imperial Gala and Fuji Suprema cultivars may be related to ethylene production, where Imperial Gala produces more than Fuji Suprema, with pyraclostrobin interfering with ethylene production. Regarding other nutrients, in the Imperial Gala cultivar an increase in K levels is observed 45 days after full flowering. As there was an influence on Ca and K levels, the N/Ca ratio, in the Imperial Gala cultivar, shows a small reduction in the four years studied, which is beneficial for the conservation of the fruits.

The number of fruits per plant in the Imperial Gala cultivar in Fraiburgo was higher in the treatment with pyraclostrobin in the four years studied, which may be related to a possible effect on increasing effective fruiting. As a result, there was an increase in production per plant, representing 36,6% of the accumulated production over the four years. Results regarding the average weight of the fruits, in the Imperial Gala cultivar, in Fraiburgo, it increased significantly in the treatment with pyraclostrobin in the years 2006 and 2007, and in 2008 and 2009 it showed lower average fruit weight, which can be attributed to the constant increases of production. It is noteworthy that even with a greater number of fruits per plant, there was an increase in the average weight of the fruits, in the treatment with pyraclostrobin in Fraiburgo in the first two years of evaluation. The physiological effects of strobilurins are cited as increasing the concentration of chlorophyll, proteins and biomass production, favoring the harvest rate. In the Fuji Suprema cultivar, the number of fruits per plant showed a small increase in the first two years and was very similar in the remaining years, a behavior also observed in the production per plant, with the average weight of the fruits being higher in the treatment with pyraclostrobin in 2006, 2008 and 2009.

The red color of the fruit epidermis did not show a significant difference in the Imperial Gala cultivar, the same occurred with regard to the incidence of russeting, which means that it can be stated that pyraclostrobin does not increase the incidence of russeting.

The vitamin C contents of the fruits were increased in analyzes carried out ten days before the harvest point and at the harvest point in the Imperial Gala cultivar (Figure 1). In samples at the point of collection, the concentration of vitamin C increased significantly in the treatment with pyraclostrobin, in apples with and without skin, which shows evidence of the effect of pyraclostrobin in increasing the concentration of vitamin C in apple fruits. In the Fuji Suprema cultivar, the same behavior was observed in the analyzes carried out at the point of harvest, however, the percentage increase was 23,96% in apples without skin, while in apples with skin it was 14,95%. Vitamin C levels were higher in the Fuji Suprema cultivar, compared to the Imperial Gala cultivar.

Regarding disease control, in both treatments the incidence was very low, however, treatment with pyraclostrobin (metiram + pyraclostrobin) reduced scab in relation to standard treatment in the Imperial Gala cultivar (venturia inaequalis) and Gala leaf spot (Colletotrichum sp.). Regarding rot in the Imperial Gala cultivar in stored fruits, there was a small reduction in the treatment with pyraclostrobin, however, this difference increased in relation to the standard treatment when the fruits were evaluated seven days after removal from the cold room.

CONCLUSIONS

1.Pyraclostrobin (metiram + pyraclostrobin) increased the vitamin C content in the pulp of apple fruits.

2. Calcium levels in fruit pulp increased during treatment with pyraclostrobin, being more pronounced in the Imperial Gala cultivar.

3. With the increase in Ca levels, the N/Ca ratio was reduced in treatment with pyraclostrobin.

4. The K content of the fruit pulp was increased in treatments with pyraclostrobin, however, in some cases without significant differences.

5.The incidence of russeting and the red color of the fruit epidermis were not influenced by pyraclostrobin treatment.

6. The average weight of the fruits and the number of fruits per plant showed variable results, requiring further studies;

7. In treatment with pyraclostrobin, there was a lower incidence of scab and Gala leaf spot in treatment with pyraclostrobin.

Tabela 1 – Calcium content (mg kg^-1) in the pulp of apple fruit cultivars Imperial Gala and Fuji Suprema in the pre-harvest period under two phytosanitary management systems. Fraiburgo (SC), 2010

Cycle	CV. Imperial Gala		CV. Fuji Supreme
	Driving pattern	Driving Pyrocl.	Driving pattern	Driving Pyrocl.
2006/2007	42,3	49,9	36,6	35,2
2007/2008	36,7	38,2	42,1	43,1
2008/2009	64,3	76,5	48,3	51,3
2009/2010	44,6	52,3	63,6	64,4

Cycle

CV. Imperial Gala

CV. Fuji Supreme

Driving

pattern

Driving

Pyrocl.

Driving

pattern

Driving

Pyrocl.

2006/2007

42,3

49,9

36,6

35,2

2007/2008

36,7

38,2

42,1

43,1

2008/2009

64,3

76,5

48,3

51,3

2009/2010

44,6

52,3

63,6

64,4